1. Field of the Invention
The present invention relates to a photographic element containing a light-sensitive silver halide emulsion with enhanced photographic speed.
2. Description of the Related Art
Various techniques have been used to improve photographic speed of silver halide photosensitive materials. Chemical sensitizers such as compounds of sulfur, gold and metal of the VIII group are used to enhance the inherent sensitivity of silver halide. Further, spectral sensitization using cyanine and other polymethyne dyes is also a technique well known in the field.
It is well known as dye desensitization that the photographic speed is remarkably reduced by addition, to an emulsion, of a spectral sensitizing dye exceeding its optimum amount. As a method of solving this problem, a technique of utilizing supersensitizing effect of a supersensitizer. Supersensitizers are generally colorless organic compounds, which have an effect of acting on sensitizing dyes (or excited sensitizing dyes) to inhibit dye desensitization.
Examples of such compounds are shown in the following patents: U.S. Pat. Nos. 2,937,089, 3,706,567, 2,875,058, 3,695,888, 3,457,078, 3,458,318, 3,615,632, 5,192,654, 5,306,612, 2,419,975, 5,459,052, and 4,971,890, and EP 554856.
Further, various electron-donating compounds are used together with sensitizing dyes to enhance the spectral sensitivity of silver halide photosensitive materials. Examples thereof are disclosed in U.S. Pat. Nos. 3,695,588 and 3,809,561, and GB's 255084 and 1064193.
Furthermore, used is a compound obtained by binding the electron-donating compound to a sensitizing dye by a covalent bonding. Examples thereof are disclosed in U.S. Pat. Nos. 5,436,121 and 5,478,719 (a compound having an electron-donating styryl base bound to a monomethyne dye), U.S. Pat. No. 4,607,006 (a compound having a triarylamine skeleton bound to an electron-donating group derived from phenothiazinephenoxazine, carbazole, dibenzophenothiazine, ferrocene, and tris-2,2′-bipyridyl-ruthenium, or to a silver halide adsorptive group).
However, even by adopting the above means, an ideal high photographic speed has not yet been realized. In particular, in the present circumstances there are few compounds which can achieve high speed and simultaneously overcome a problem of fog caused by increase in speed, and a problem of storage fog caused by storage under sever conditions, for example, silver halide photosensitive material is kept under a high temperature and a high humidity or exposed to noxious gas generated by combustion, such as car exhaust.
In the meantime, recently, a sensitizing technique using compounds called “two-electron sensitizers” which are fragmentized (bond cleavage) after being one-electron-oxidized and can further release one electron has been reported, as shown in U.S. Pat. Nos. 5,747,235, and 5,747,236, EP's 786692A1, 893731A1, and 893732A1, WO99/05570, and a thesis appearing on a U.S. chemical society journal, “Two-Electron Sensitization: A New Concept for Silver halide Photography”, J. Am. Chem. Soc., 122, 11934-11943 (2000). The documents describe that the compounds are characterized in that the compounds are oxidized by a dye positive hole (a sensitizing dye molecule which has lost one electron, after the electron has been injected from the sensitizing dye in an excited sate into a conduction band of silver halide), or a positive hole generated by excitation of silver halide, and then release another electron only after reaction of fragmentation, which provides high sensitivity.
However, even these compounds cannot achieve an ideal high speed which can provide a photosensitive material having a high speed/fog ratio and an excellent storability.